Much of my students' work will show the presence of pentimenti (erased marks left behind as corrections are made). Making mistakes and correcting them is natural to the drawing process. Many students don't realize this when they start out, experiencing an unnecessary amount of frustration when they don't "get it right" the first time. Later they will realize that this pentimenti can contribute the beauty and rhythm of the piece, and that it can stand as a touching testament to the humility of artists, even old masters, who show that they made errors.